IP/aiki provides an edge but it is not by any means complete on its own. Training for fighting requires other attributes as well, such as cardiovascular conditioning as well as raw strength. Both of those feed into and support the combative mindset, since without proper conditioning you gas out. As Vince Lombardi said, fatigue makes cowards of us all.
Layered on that is strategy for particular engagement parameters. As Kevin Leavitt said, it's about controls and parameters.
Let's take a look at this from a sport fighting point of view: judo is different from muay thai, which is different from sanda. To be really good at one, you have to optimize your training program. Even things that seem similar, like muay thai and sanda actually have different rules and therefore different strategies for victory. Last year I witnessed a match between two heavyweights under sanda rules. One of the fighters had fought on a K1 undercard. He lost. Different rules (parameters), and he fought a guy with a lot of experience in that ruleset. The fighters weren't allowed to throw knees, and it looked like his inside/clinch strategy was dependent on that. I should add that I have personally lost a match because of failure to adapt to a particular ruleset, and lulling myself into a false sense of confidence by improperly structuring my training.
Saying that IP/IT is THE important thing misses out on the fact that there are a lot of things that go into making a successful fighter. A person could have all the IS/IP they want and still get knocked out by a straight right hand if they don't know how to move their head or cover appropriately.
If you reread my post, you'll note that I state " While those other things are important and are things that people can learn". Perhaps I should have put in bold " While those other things are important
and are things that people can learn
"? I really didn't know how to make that any clearer.
As for "THE". What was it that made Takeda, Ueshiba, Sagawa, Horikawa, etc all stand out from the rest of the other jujutsu guys? All the rest of the fighters? Boxers? Wrestlers? Etc? What was THE single thing that made them stand so far out from everyone else?
I had already stated that training in other things was important. If you wanted to be a judoka, you trained in judo. If you wanted to be a boxer, you trained for boxing. etc. But why was it that all these experienced martial artists with many years of training, pretty much all stated that Takeda, Ueshiba, Sagawa, Horikawa, etc were very different and doing something completely alien to them? Something that they either could not negate or had an extremely hard time doing so?
If training in judo, jujutusu, kenjutsu, sumo, boxing, fighting, etc were "THE" important thing, why didn't everyone who had 40 years training experience think Takeda, etc were all doing something that they understood and could do? Why couldn't they handle Takeda, etc?
That's not saying IP/aiki is the be all/end all to everything. We were comparing what was important regarding training and assigning priorities to training. In that regard, aiki is still THE important thing to train.